Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for performing a marine seismic survey using autonomous underwater vehicles (AUVs) that carry appropriate seismic sensors and use an acoustic modem to communicate with a vessel for arriving at a target location on the ocean floor.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration to determine the properties of earth's subsurface, which is especially helpful in determining the above-noted reservoirs. Marine reflection seismology is based on using a controlled source of energy that sends energy into the earth. By measuring the time it takes for the reflections and/or refractions to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A traditional system for generating seismic waves and recording their reflections off the geological structures present in the subsurface is illustrated in FIG. 1. A vessel 10 tows an array of seismic receivers 11 provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to the ocean surface 14, or they may have other spatial arrangements. The vessel 10 also tows a seismic source array 16 configured to generate a seismic wave 18. The seismic wave 18 propagates downward toward the seafloor 20 and penetrates it until a reflecting structure 22 (reflector) eventually reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by the receiver(s) 11 on the streamer(s) 12. Based on the data collected by the receiver(s) 11, an image of the subsurface is generated.
However, this traditional configuration is expensive because the cost of streamers is high. New technologies deploy plural seismic sensors on the bottom of the ocean (ocean bottom stations) to improve the coupling. Even so, positioning seismic sensors remains a challenge.
A new technology employs probes that are launched from a vessel and, by virtue of gravity, travel to the ocean bottom. However, these probes do not possess an active navigation system and, thus, the landing accuracy is poor, i.e., the probes do not land at the intended location. This inaccuracy in the association of the recorded seismic data with the probe's geographical location propagates to the processing stage, resulting in a degraded image of the subsurface being surveyed. Another disadvantage of this technology is that the probes leave behind corresponding pedestals when they are instructed to resurface, which are heavy pieces of equipment that make good contact with the ocean floor. This contributes to ocean pollution, which is undesirable.
Accordingly, it would be advantageous to provide inexpensive and non-polluting systems, methods and devices for reaching the seafloor, recording seismic waves and resurfacing for data transfer.